The intervertebral disc functions to stabilize the spine and to distribute forces between vertebral bodies. The intervertebral disc is composed primarily of three structures: the nucleus pulposus, the annulus fibrosis, and two vertebral end-plates. These components work together to absorb the shock, stress, and motion imparted to the spinal column. The nucleus pulposus is an amorphous hydrogel in the center of the intervertebral disc. The annulus fibrosis, which is composed of highly structured collagen fibers, surrounds and constrains the nucleus pulposus within the center of the intervertebral disc. The vertebral end-plates, composed of hyalin cartilage, separate the disc from adjacent vertebral bodies and act as a transition zone between the hard vertebral bodies and the soft disc.
The nucleus pulposus typically contains a negatively charged proteoglycan component. Proteoglycans are glycoproteins with many polysaccharide side chains, and have properties that are more like typical polysaccharides than proteins. The proteoglycan component of the nucleus pulposus associates with water to form a hydrated gel. Water may reach the nucleus pulposus from sieve-like pores in the end plates. The resulting osmotic pressure within the intervertebral disc causes it to expand axially (i.e., vertically), driving the adjacent vertebrae apart. On the other hand, mechanical movements resulting in axial compression, flexion, and rotation of the vertebrae exert forces on the intervertebral disc, which tends to drive water out of the nucleus pulposus. Water movement into and out of an intervertebral disc under the combined influence of osmotic gradients and mechanical forces is important for maintaining disc health. In a normal healthy nucleus pulposus, water comprises between about 80% to about 90% of the nucleus's total weight.
Intervertebral discs may be displaced or damaged due to trauma, disease, and the normal aging process. Intervertebral discs undergoing degeneration typically experience dehydration relatively early in the degeneration process. During dehydration of the intervertebral disc, the water associated with the proteoglycan hydrogel comprising the nucleus pulposus of the disc may be lost. Dehydration of the nucleus may result in collapse of the disc space and reduced disc space height. Reduced disc space height may lead to instability of the spine, decreased mobility, and back and leg pain.
Several general strategies have been proposed in order to restore disc height in a dehydrated intervertebral disc. In one strategy, the disc is treated by inducing repair or regeneration of the nucleus with a biological treatment. Biological treatments include a broad variety of treatment regimens such as the implantation of nucleus pulposus cells harvested from healthy intervertebral discs, steroidal injections to induce cell proliferation, genetic treatments to induce and/or increase proteoglycan production by the pulposus cells, and so forth. Unfortunately, effective biological treatments appear to be many years away from commercial development and routine use.
In another strategy to treat dehydrated intervertebral discs, a portion or all of the nucleus is removed and a prosthetic nucleus device is implanted in the intervertebral disc space to augment or completely replace the dehydrated nucleus. Alternatively, a total disc replacement (“TDR”) operation may be performed wherein not just the dehydrated nucleus but the entire intervertebral disc is removed and replaced with a prosthesis. However, nucleus and TDR replacements remain unproven. Also, even when minimally invasive surgical techniques are used, these surgeries are relatively difficult to perform and inflict a good deal of trauma on the patient, resulting in increased post-surgical recovery times and disability. Additionally, the complexity of currently available intervertebral prostheses necessitates careful and meticulous consideration of the patient's unique prognosis to determine which prosthesis is most likely to result in a positive therapeutic outcome.
The description herein of problems and disadvantages of known apparatuses, methods, and devices is not intended to limit the embodiments to the exclusion of these known entities. Indeed, embodiments may include one or more of the known apparatus, methods, and devices without suffering from the disadvantages and problems noted herein.